As is known a friction clutch disc comprises an annular support plate which may be circumferentially divided up into a plurality of segments, and two annular friction facings disposed on opposite sides along an outer peripheral region of the annular plate which may also be divided into segments, each one of the friction facings being fixed to the plate segments at spaced locations by fixing means.
The annular plate is fitted with a hub, either directly or by the medium of a torsion damping assembly. Such a clutch disc is adapted to be mounted on a shaft and in a motor vehicle it is normally the transmission input shaft.
By means of the friction facings the clutch disc is adapted to be clamped for rotation with another shaft, axially between two places fixed for rotation with the other shaft which in a motor vehicle is normally the engine output shaft.
In any event the means for fixing the friction facings to the annular support plate are most commonly rivets.
The rivets alternately secure each one of the friction facings, accommodated in alignment with recesses formed in the other of the friction facings for assembly.
Each of the rivets comprises, in practice, a shank which extends substantially axially through a bore formed in the friction facing that is to be fixed to the annular support plate and through an opening in the support plate. The rivets further comprise a first head at one end which bears against a generally transverse shoulder at the axially inner end of the bore in the friction facing and a second head at the other outer end of the shank which bears against the annular support plate.
Such an arrangement is for example disclosed, in combination with other features, in U.S. Pat. No. 3,921,777 assigned to the assignee of the present application.
In practice, for the sought after securement of the friction facings, each of the rivets necessarily clamps under its first head, and between the first head and the annular support plate, a not insubstantial thickness of the friction facing being secured, which thickness depends on the upsetting distance to take into account.
This thickness which up to the present has run from the face of the support plate against which the friction facing to be fixed is applied together with the thickness of the first head defines a lost thickness or unusable part of the facing. The effective thickness of the friction facing is reduced correspondingly. The maximum possible wear of such a friction facing is limited to the axially outer part of the friction facing down to the head of the corresponding rivet.
For this reason friction facings are typically extra thick with respect to the thickness which is actually required for wear. Consequently, the friction facings require a surplus friction material for their manufacture, and in operation their superfluous rotating mass produces additional inertia which may cause excessive fatigue of at least some of the components arranged after the clutch disc along the kinematic chain and in particular synchronizers which customarily equip the associated gearbox, to the detriment of the service life thereof.